Electrical earth borehole logging is well known and various devices and various techniques have been described for this purpose. Broadly speaking, there are two categories of devices used in electrical logging devices. In the first category, a transmitter (such as a current electrode) is uses in conjunction with a diffuse return electrode (such as the tool body). A measured electric current flows in a circuit that connects a current source to the transmitter, through the earth formation to the return electrode and back to the current source in the tool. In inductive measuring tools, an antenna within the measuring instrument induces a current flow within the earth formation. The magnitude of the induced current is detected using either the same antenna or a separate receiver antenna. The present disclosure belongs to the first category.
With tools in the first category, it is desirable for the current to penetrate the earth formation to at least a minimum desired depth before returning through the return electrode. One method of preventing early current return, and bypassing most or all of the earth formation, is to use a biasing element. A biasing element, which may be a metallic structure capable of carrying an electric potential, may be energized to a specified voltage so as to prevent an undesired electric current return. However, this biasing method is not perfect, and substantial amounts of the source current may travel into the biasing element (current leakage) rather than to the return electrode. This current leakage interferes with measurement of the resistive properties of the formation. This disclosure addresses mitigation of this current leakage.